1. Field of the Invention
The present invention relates generally to the fabrication of integrated circuits on substrates. Specific embodiments of the invention relate to methods and apparatus for adjusting electrochemical baths used for electrochemical deposition processes.
2. Background of the Invention
Sub-quarter micron, multi-level metallization is one of the key technologies for the next generation of ultra large-scale integration (ULSI). The multilevel interconnects that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio apertures, including contacts, vias, lines and other features. Reliable formation of these interconnect features is very important to the success of ULSI and to the continued effort to increase circuit density and quality on individual substrates and die.
As circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to less than 250 nanometers, whereas the thickness of the dielectric layers remains substantially constant, with the result that the aspect ratios for the features, i.e., their height divided by width, increases. Many traditional deposition processes, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), have difficulty filling structures where the aspect ratio exceed 4:1, and particularly where it exceeds 10:1. Therefore, there is a great amount of ongoing effort being directed at the formation of void-free, nanometer-sized features having high aspect ratios wherein the ratio of feature height to feature width can be 4:1 or higher.
Currently, copper and its alloys have become the metals of choice for sub-quarter-micron interconnect technology because copper has a lower resistivity than aluminum, (1.7 μΩ-cm compared to 3.1 μΩ-cm for aluminum), a higher current carrying capacity, and significantly higher electromigration resistance. These characteristics are important for supporting the higher current densities experienced at high levels of integration and increased device speed. Further, copper has a good thermal conductivity and is available in a highly pure state.
Despite the desirability of using copper for semiconductor device fabrication, choices of methods for depositing copper into features having high aspect ratios, such as a 10:1 aspect ratio, 0.25 μm wide vias, are limited. In the past, chemical vapor deposition (CVD) and physical vapor deposition (PVD) were the preferred processes for depositing electrically conductive material into the contacts, vias, lines, or other features formed on the substrate. However, for copper applications, CVD processes are limited to the use of copper containing precursors, which are still being developed, and PVD processes have faced many difficulties for depositing copper conformally in very small features. As a result of the obstacles faced in PVD and CVD copper deposition, electrochemical deposition, which had previously been limited to circuit board fabrication, is being used to fill high aspect ratio features of substrates.
Electrochemical deposition can be achieved by a variety of techniques, such as by electroplating or electroless deposition. In an electroplating deposition, conductive materials are deposited over a substrate surface by chemical reduction in the presence of an external electric current. In particular, electroplating uses a solution, often referred to as an electrochemical bath, of generally positively charged ions of the conductive material, such as copper, to be deposited in contact with a negatively charged substrate of conductive material. The negatively charged substrate provides an electrical path across the surface of the substrate, where an electrical current is supplied to the conductive material to reduce the charged ions and deposit the conductive material. A variety of electrochemical baths may be used, each having different chemical compositions comprising various ingredients or components (hereinafter “constituents”) of variable concentrations.
Electrochemical baths may also be used for an electroless deposition of a conductive material. In an electroless deposition, the conductive material is generally provided as charged ions in an electrochemical bath over a catalytically active surface to deposit the conductive metal by chemical reduction in the absence of an external electric current. The electroless process provides selective deposition of the conductive materials at locations where a catalytic material already exists. The electroless process is self-perpetuating to the extent of the availability and composition of the electroless deposition solution and other reactive conditions. Descriptions of the electroless deposition process in Chapter 31 of Modern Electroplating, F. Lowenheim, (3d ed.) and in U.S. Pat. No. 5,891,513 are incorporated herein by reference to the extent not inconsistent with the invention.
Providing optimal electrochemical bath compositions is important in sub-micron conductive material deposition applications and volume production of microelectronic devices. One approach to conditioning the electrochemical bath composition during substrate to substrate processing is to analyze the electrochemical bath periodically during the plating process to determine the composition and concentration of the constituents in the electrochemical bath. Then the results of the analysis may be used to adjust the composition of the electrochemical bath by adding constituents that have been consumed during processing of the electrochemical bath.
However, the above described approach has certain deficiencies. Not only is it difficult to reconstitute the initial bath composition, but it has been discovered that the composition of the electrochemical baths will also vary over time. In some instances, an electrochemical bath formed during a deposition process will produce higher quality films than films deposited under the initial processing conditions. For example, the deposition performance of copper is enhanced in the area of grain growth control and management near the “end of life” of the bath than compared to the initial electrochemical bath, often referred to as the “beginning of life” of the electrochemical bath. The “end of life” of the bath is defined as when the one or more constituents of the electrochemical bath have been depleted during the deposition process. Therefore, it is highly desirable to determine the preferred concentration of the constituents of the electrochemical bath under later processing conditions, and to further maintain or produce those processing conditions to produce high quality depositions that are consistent from substrate to substrate. Currently, there is no effective way of maintaining or producing the preferred electrochemical bath compositions that occur under later processing conditions, for example, at or near the “end of life” of the electrochemical bath for copper deposition.
Therefore, there remains a need for a process and apparatus for analyzing and conditioning electrochemical baths.